Spatial multi-zone sound field reproduction over an extended region of space has recently drawn increased attention due to its various applications such as simultaneous car entertainment systems, surround sound systems in exhibition centers, personal loudspeaker systems in shared office space, and quiet zones in a noisy environment, where the aim is to provide listeners an individual sound environment without having to use acoustical barriers or headphones. Corresponding systems are also referred to as personal audio or private sound zone (PSZ) systems.
Generally, a sound field can be considered to describe the deviations of the local air pressure from the ambient pressure, i.e. the pressure variations, as a function of space and time caused for instance by the sound signals emitted by a plurality of loudspeakers. A multi-zone sound field usually can comprise one or more acoustically bright zones and possibly several acoustically dark zones as well as grey zones.
Known systems for personal audio are generally based on a performance trade-off between directivity, input energy required by the loudspeaker array to perform directional sound radiation, and accuracy of reproduction of the desired sound field in the listening area, hereafter succinctly referred to as quality. For example, a given system for personal audio may be able to provide high directivity at the expense of a reduced quality in the listening zone, as described, for instance, in the article “Controlled sound field with a dual layer loudspeaker array” by Mincheol Shin, Filippo M Fazi, Philip A Nelson, and Fabio C Hirono, J. Sound Vib., 333(16):3794-3817, August 2014 (hereinafter referred to as Shin et al).
A widely used signal processing method for the design of the input signals to the loudspeaker array is the Pressure-Matching (PM) method. A more general formulation of the PM method is the Weighted-Pressure Matching (WPM) method, which has been used in a number of implementations of known systems for personal audio. In the WPM method, appropriate tunable parameters can be used to design the input signals that provide a desired performance trade-off.
A number of methods have been proposed to control this trade-off that are based on the WPM, such as those proposed in the following articles: Ji Ho Chang and Finn Jacobsen, “Sound field control with a circular double-layer array of loudspeakers”, J. Acoust. Soc. Am., 131(6):4518, June 2012; Terence Betlehem and Paul D. Teal, “A constrained optimization approach for multi-zone surround sound”, in 2011 IEEE Int. Conf. Acoust. Speech Signal Process., volume 1, pages 437-440. IEEE, May 2011; Yefeng Cai, Ming Wu, and Jun Yang, “Sound reproduction in personal audio systems using the least-squares approach with acoustic contrast control constraint”, J. Acoust. Soc. Am., 135(2):734-741, February 2014 as well as the article by Shin et al.
The methods proposed by Chang et Jacobsen and Shin et al. can be considered as “fixed-value parameter” methods, because, in their original formulations, the tunable parameters can be set by the user. The methods proposed by Betlehem and Teal and Cai et al. include on the other hand algorithms for an iterative calculation of the optimal parameters. In this case, these can be referred to as “iterative” methods. The fixed-value parameter methods have the advantage of faster filter calculation (no parameters have to be calculated), but fail to provide an accurate prediction of final performance. On the other hand, iterative methods provide accurate predictions of final performance, but slower filter calculation.
Current systems for private sound zones are designed for a fixed, pre-defined scenario. However, often it might be desirable that a user can rapidly change a scenario. For instance, for a single listener located at a specific point in a given environment, where other people are present, it might be desirable to have a better audio quality as opposed to a highly directive sound, or to change the scenario, i.e. the location and number of the private audio zones.
Thus, there is a need for improved apparatuses and methods for generating a sound field allowing, in particular, for a flexible adaption of the sound field scenario as well as a desired directivity and quality trade-off.